The correct answer is: A. They contain free electrons.
Metals are good conductors of electricity because they contain free electrons. These electrons are not bound to any particular atom, and they can move freely throughout the metal. When a voltage is applied to a metal, these free electrons move in response to the electric field, creating an electric current.
The other options are not correct. Option B is incorrect because the atoms in a metal are not lightly packed. In fact, the atoms in a metal are very close together, which is why metals are solid at room temperature. Option C is incorrect because the melting point of a metal does not affect its ability to conduct electricity. For example, mercury has a very low melting point, but it is still a good conductor of electricity.
In conclusion, metals are good conductors of electricity because they contain free electrons. These electrons are not bound to any particular atom, and they can move freely throughout the metal. When a voltage is applied to a metal, these free electrons move in response to the electric field, creating an electric current.